The present invention relates to industrial quality control, and in particular to the industrial quality control of products with weakly structured relief-type surfaces, when checking for fabrication faults which manifest themselves as a specific anomaly in the surface shaping.
In industrial quality control a frequent task is to investigate convex surfaces for fabrication faults in situations where the convex surface exhibits deliberately created structurings in addition to any fabrication faults which may be present. One of the most important applications in this context is the detection of so-called bulges or constrictions on the tire sidewalls of vehicles. The chief difficulty in the detection of such fault features lies in the fact that the areas under test exhibit raised relief-type letters or other markings which are provided intentionally, that the letter and fault structures lie within the same height interval or that the faults to be detected can have a smaller depth than the structures introduced during manufacture, and moreover that the side surfaces under test exhibit a torus-shaped curvature. As a consequence, a simple threshold value decision on the basis of measured form data does not lead to an unequivocal distinction between faults and lettering. It should also be borne in mind that there are many tire manufacturers, each of them offering a multitude of tire types. Each tire type can have different dimensions and therefore a different tire sidewall curvature. Furthermore, each tire type will have a different inscription. While the manufacturer""s inscription is normally much the same from one tire type to another, the designation of the tire type itself and further information to be found on tires vary markedly from one type to another. Tire inscriptions are generally such that a manufacturer""s letters e.g. do not protrude from the tire sidewall but are surrounded by a raised symbol such as a triangle when seen in cross section, the symbol normally emerging sharply, i.e. with an edge, from the surface of the tire. In addition there may be other edge-delimited raised portions which, however, normally occupy relatively small areas.
In the production of vehicle tires, fabrication faults in the inner construction may arise which significantly affect the mechanical properties and thus the stability behaviour. Such fabrication faults, e.g. in the steel network around which the tire rubber is arranged, are visible on the outside as localized bulges or constrictions. Such products must be removed. These construction faults are liable to occur particularly on the sidewalls and are thus superimposed on the undisturbed torus-shaped surface of the tire. The lateral and vertical dimensions of such defective structures are comparable with the constructional ones which are also present, i.e. intentional structuring on the surface in the form of raised letters or markings.
Until now capacitive measurement methods have primarily been employed to perform these checks in the industrial sphere, but these cannot provide an adequate level of checking. During the movement of the surface, i.e. during a rotation of the tire, a change in the capacitance of the measurement sensor is used to detect a change in the distance between the measurement electrode and the surface of the tire. The distance here is of the order of 1 cm. The disadvantage of this method lies in its geometrically related relatively crude lateral spatial resolution, as a consequence of which it is only possible to measure sensibly a small number of tracks across the width of the test area. The measurement signal does not contain sufficient information on the precise geometry of a height deviation. Additionally, it is not possible to distinguish between lettering and fault structures since both structures can lie within the same height range relative to the surrounding surface. The resulting detection threshold for faults must therefore generally be chosen higher than the height of the lettering to avoid provoking permanent pseudofault detection due to constructional structures.
Despite the preponderant use of this method the industrial user is in need of a test method which permits certain fault detection for height deviations of the same order as the size of the constructional lettering or marking structures.
A known method for measuring surface contours is optical triangulometry, also called the light intersection method. Here a narrow light beam, generally a laser beam, is directed onto the surface to be measured and the diffusely reflected radiation is optically mapped onto a sensor having a number of picture elements, i.e. pixels. If the geometry between the camera and the light beam remains unchanged, the change in the spatial position of the light intersection point on the measured object along the beam can be calculated from a shift in the projected light point on the sensor surface.
In the first instance such a measurement is for a point. If a whole region is to be checked, the test object is moved under the triangulator measurement setup from one side to the other and the measured values are recorded rapidly so that a small circular structure on the tire sidewall is detected.
A disadvantage of this method, however, is that up to now it is not possible by means of post-circuited data processing to distinguish between the fault structures, i.e. bulges and constrictions, and the raised lettering. With this method fault structures can be detected only for heights which markedly exceed that of the lettering, so that, apart from the improved resolution in the radial direction, there are no advantages worth mentioning compared with the capacitive method.
A further development of the light intersection method is achieved by scanning the surface with a fan beam and an area sensor. The light line projected onto the sensor can be used to determine the height information along the line of measurement on the convex surface. By moving the object the height information is recorded line by line and is then combined to form a complete data record. The data record created in this way contains the height information from a whole surface region, including the faults and the lettering and marking structures. However, since lettering and fault structures lie within the same height range and these structures also lie on a surface which is strongly curved compared with these structures, it has not yet been possible, using the data thus acquired, to achieve certain distinction, by means of a threshold value decision, between actual fault irregularities and the intentional structuring in the form of lettering and marking.
DE 44 42 980 C2 discloses a device and a method for the frictionless detection of the contour of a convex surface, wherein a unit creates a three-dimensional representation of the surface.
DE 43 04 451 A1 discloses a device and a method for the frictionless detection of an irregularity, such as a seam e.g., also in a convex surface, e.g. rubber layers used in the construction of tires, wherein a unit creates sectional views of the surface.
U.S. Pat. No. 4,402,218 discloses a device and a method for detecting a potentially existing essentially edge-free irregularity in a convex surface, wherein a unit creates a representation of the surface.
DE 39 31 132 A1 discloses a device and a method for detecting irregularities, such as a rough patch e.g., on a convex surface, wherein one unit creates data representing the surface and another unit extracts the convexity from this representation.
DE 38 01 297 A1 discloses a method for detecting surface irregularities and creating data in digital form as a representation of the surface; furthermore the shortwave structurings are smoothed by appropriate damping of the associated harmonic components of the surface curve and a representation of the surface is created which comprises the longwave irregularities, the shortwave ones being smoothed.
It is the object of the present invention to provide a device and a method for the contactless detection of an essentially edge-free irregularity in a convex surface, which has a structuring that is delimited by edges, which enable a better distinction to be made between faulty irregularities and the edge-delimited structuring.
In accordance with a first aspect of the present invention, this object is achieved by a device for the frictionless detection of a potentially existing essentially edge-free irregularity in a convex surface, which has a structuring that is delimited by edges, comprising: a unit for creating a three-dimensional representation of the surface; a unit for extracting the convexity from the three-dimensional representation of the surface and for smoothing the edges of the structuring so as to obtain a convex-free representation of the convex surface which exhibits the potentially existing irregularity and the structuring, whose edges have now been smoothed; a unit for comparing the convex-free representation with a threshold so as to identify areal regions of the convex-free representation which are determined by a predetermined relationship to the threshold value; and a unit for analyzing the areas of the identified regions, a region being detected as an irregularity if its area exceeds a predetermined area.
In accordance with a second aspect of the present invention, this object is achieved by a method for the frictionless detection of a potentially existing essentially edge-free irregularity in a convex surface, which has a structuring that is delimited by edges, comprising the following steps: creating a three-dimensional representation of the surface; extracting the convexity from the three-dimensional representation of the surface and smoothing the edges of the structuring so as to obtain a convex-free representation of the convex surface which exhibits the potentially existing irregularity and the structuring, whose edges have now been smoothed; comparing the convex-free representation with a threshold so as to identify areal regions of the convex-free representation which are determined by a predetermined relationship to the threshold value; and analyzing the areas of the identified regions, a region being detected as an irregularity if its area exceeds a predetermined area.
The present invention is based on the finding that, although the irregularity in a convex surface and the structuring may have the same height, they differ markedly in their delimitation. The irregularity, e.g. in the form of a constriction or a bulge on one of the tire sidewalls, exhibits an essentially edge-free, i.e. continuous, transition to the undisturbed convex surface. On the other hand a structuring, which is deliberately imposed on the convex surface, is characterized by the fact that it is delimited by edges, i.e. that the transition from the convex surface to the structuring is abrupt and sharply delimited. According to the present invention, therefore, the specific differences between the two form types is exploited, after three-dimensional detection of the convex surface, to achieve a selective smoothing of the structuring without influencing the irregularities which are to be detected. Based on the fact that the irregularities which are to be detected are locally delimited on the convex surface, the convexity of the convex surface is also extracted without influencing the irregularity or the structuring.
In this way a convex-free representation of the convex surface is obtained which exhibits the irregularities which are to be detected and the structuring, the edges of which are smoothed, however. What is involved here is, in a way, a quasi-planar post-processed representation of the tire sidewall, which can be subjected to a threshold value decision so as to mark areal regions whose height lies above or below the threshold value. Through a subsequent analysis of the areas of the detected regions, irregularities are separated from any xe2x80x9cremainsxe2x80x9d of the structuring, which may still be present in some circumstances, when the area of the irregularity determined by the threshold value decision is greater than a predetermined area.
The edge-delimited structuring is not completely eliminated by the post-processing. Instead, the edges are simply smoothed, the height of the structuring also being affected as a result of the smoothing operation. In consequence the region which remains from a structuring after the threshold value is considerably reduced if not completely suppressed by the data processing while the essentially edge-free irregularities are scarcely affected if at all. As a result an analysis of the areal regions after the threshold value decision in relation to a predetermined area enables an irregularity to be distinguished from a structuring to a high degree of accuracy regardless of whether the height or the extent of the structuring is close to the height and extent of the bulges or not.
A preferred application of the concept according to the present invention relates to the quality control of tires where the tire sidewalls are checked for bulges or constrictions. A constriction, i.e. depression in relation to the undisturbed convex surface, can always be assumed if an areal region is detected which lies below a negative height threshold value, since tires do not normally exhibit structurings which penetrate into the tire sidewall.
The structurings normally inscribed on tires are so constituted that letters or other data are represented by means of prominent borders with relatively small cross-sections. Insofar as these borders are not already completely brought below the threshold value by the smoothing, they will give rise to only very small areas in the analysis of the areas of the detected regions, thus enabling a simple distinction to be made between the structuring and the bulges since bulges will normally have a relatively large dimension. The device and the method according to the present invention thus permit automatic quality control of tire sidewalls which can be performed in real time if the cameras for registering the tire and the image processing architecture are fast enough.